The present invention relates to an electrical apparatus fir charging a battery from a voltage grid and further to a method for charging a battery from a voltage grid. The apparatus is suitable for electric vehicles and other machines comprising an electrical machine.
The share of partly or only electrically driven vehicles is expected to increase radically in the next 10 to 20 years. This also means a corresponding increase in the need to charge such vehicles. In principle, the charging is done by connecting the vehicle to the electrical network, either directly to one of the ordinary outlets used for other equipments, or through a special charger, external to the vehicle. Charging current transferred from the network to a battery on board the vehicle. In the latter case with an external charger, standardization is needed so that vehicles from different manufacturers can all be charged from the same type of outlet.
A passenger car uses around 2 kWh of electrical energy per 10 km for its propulsion. An ordinary single-phase outlet is, in some markets, fuse-protected with 10 A, which limits the power transfer to about 2 kW at 230 V. This means that a vehicle which is charged from such an outlet receives energy for around 10 kilometers of driving each hour it is connected to the charge. Heavier vehicles use even more energy per kilometer.
To increase the charging power it is possible to use a 3-phase outlet. These are found in many locations, fuse-protected with e.g. 16, 32 and 63 A, and possibly even higher amperages. The connectors used are strictly standardized but already for 63 A they are so large and cumbersome to handle that one cannot expect any normal person, perhaps wearing good clothing, to manually handle the connecting process. Hence 32 A could be considered to be an upper practical limit on the current level which a manually operated standard 3-phase contact device can utilize, but in some cases 63 A connectors can also be considered. With special purpose outlets and connectors, higher current connections can also be expected to be manually handled. It should be noted that already 32 A is considerably more than the fuse protection of a normal house. At 400 V 3-phase 32 A. the active power is 22 kW, i.e. a passenger car connected to such a charging outlet will receive energy for about 100 kilometers of driving for each hour it is connected fir charging. Charging times for electric vehicles are thus significantly longer than when conventional vehicles are fuelled with typically a 1000 kilometers driving range in 5 minutes. A relatively large portion of a hypothetical electrical vehicle fleet will probably be charged at night from ordinary 10 A or 16 A single phase outlets, which still gives a relatively long driving distance during a night of 10 hours charging. With a large share of electrical vehicles, however, it is likely that many of them will need to be fast-charged also in the daytime, e.g. with 22 kW as from a 400 V 3-phase 32 A outlet. Since the charging time for these still will be relatively long (typically 30 to 60 minutes), it probably does not make sense to have special charging outlets, since very many of them would be needed as compared to today's petrol stations. Instead, it is useful if these vehicles can be charged from an ordinary outlet, e.g. ranging from 230 V, 10 A 1-phase to 400 V 63 A 3-phase, or even higher currents if suitable plugs are used. These can be installed at very low costs at very many locations, e.g. parking structures in residential districts, shopping centres, offices, industrial sites, etc.
In order for a vehicle to make use of an ordinary electrical outlet, it is necessary that the vehicle itself carries the necessary charging equipment.
With a rated charging power of up to 400 V, 63 A 3-phase, providing galvanic isolation, a rather heavy (>100 kg, if a 50 Hz transformer is located on board to provide the galvanic isolation) and costly (>10,000 SEK) equipment is needed on board. Since an electric vehicle is costly already from the outset, especially on account of the costs of the batteries, it is burdened with additional costs for the charger requirement. Therefore, an integrated charging, i.e. a charging method using traction hardware, is an interesting solution to provide on-board chargers. It is known how to configure an electrical machine to operate as a transformer or commutation inductance and how to use it in combination with the power electronics to charge the battery.
One example of such a design consisting of a divisible motor winding is exhibited and described in JP10248172. One part of the motor winding can be connected to the network during charging. The design calls for a single-phase connection and driving with so-called “common mode” current, that is, the same current in all three phases of the motor.
In U.S. Pat. No. 5,341,075 there is disclosed a combined motor power and battery charging system. Two of the motor's three phase windings are used as inductors during single-phase charging of an electric vehicle battery. One requirement is that the battery voltage be higher than the highest instantaneous value of the network voltage. A significant disadvantage is that no galvanic isolation can be provided between the network the and battery. A similar solution is described in U.S. Pat. No. 5,099,186. However, two totally separate motor windings are used. Yet another solution is disclosed in U.S. Pat. No. 4,920,475.
In accordance with the above described prior art, galvanic isolation is not possible in connection with the charging of the batteries. Another drawback is that the power made available for the charging is too low. The main drawback of previously known non-isolated battery chargers is the need for a heavy and expensive Common Mode filter in order to limit common mode currents and to maintain proper grounding and safety when touching the vehicle during charging. The main drawback of previously known isolated battery chargers is low efficiency, which is to a large extent related to the need for a magnetization current.
WO 2011/159241 describes an integrated motor drive and charger based on a permanent magnet synchronous machine, in which galvanic isolation is obtained by using an electric machine with two stator windings. Both stator windings are connected in series during traction mode. However, during charging mode the stator windings are reconfigured with a switch device into a motor/generator set, which is also referred to as a rotating transformer. The rotor needs to rotate at synchronous speed (=grid frequency) during charging mode. The maximum charging power of this arrangement is below half of the maximum continuous electrical machine power. This allows for galvanic isolation at a high efficiency level of power conversion. One drawback is that the rotor must be mechanically disconnected from the rest of the system in order to be able to rotate during charging. In an electric vehicle, the clutch must thus be disengaged. Further, a relatively complicated switching device is necessary for reconfiguration of the windings.
Hence, there is a need for an improved charging arrangement for an electric machine using the electrical traction drive components.
It is desirable to provide an improved arrangement for charging a battery in a system comprising an electrical machine having two separate stator windings. It is also desirable to provide an improved method for charging a battery in a system comprising an electrical machine having two separate stator windings.
In an electrical apparatus for charging a battery, comprising an electrical drive system and an electrical machine, wherein the electrical machine comprises a rotor, a first separate multi-phase stator winding and a second separate multi-phase stator winding, wherein the drive system comprises a first multi-phase bridge inverter connected to the first multiphase stator winding and which is adapted to be connected to a grid/line voltage supply by a connection means when charging a battery, the apparatus further comprises a second multi-phase bridge inverter connected to the second multiphase stator winding and to the battery. The connection means between the first multi-phase inverter and the grid comprises a controlled or non-controlled rectifier and filter components, either passive or active.
By this first embodiment of the electrical apparatus for charging a battery according to the invention, a method for charging a battery by using the existing electrical machine and the existing drive components for e.g. driving an electric vehicle is obtained. The electrical machine comprises a rotor and two separate multi-phase stator windings and the drive system comprises two multi-phase bridge inverters, where one multi-phase bridge inverter is connected to one multi-phase stator winding and which is also adapted to be connected to a rectified line voltage supply in order to supply charging voltage to the system. The other multi-phase bridge inverter is connected to the other multi-phase stator winding and also to the battery that is to be charged. During charging, when energy is supplied from the line voltage supply, the rectifying means converts the line voltage to a predominantly DC voltage, the first multi-phase bridge inverter will drive the first stator winding such that the electric machine rotates with a predefined rotational speed. Since the stator winding is powered through the multi-phase bridge inverter, the rotational speed can be adapted to the optimal speed of the electrical machine for the charging power level intended, i.e. to the rotational speed at which the electrical machine is the most efficient. For traction motors, the optimal speed for charging is usually higher than the frequency of the line supply and hence, charging at the line supply frequency decreases the maximum charging power and eliminates the chances to select the optimum speed.
Normally, the frequency of the line voltage is low, 50 or 60 Hz, and the electric machine is designed to operate in a frequency range from zero to several hundred Hertz. By creating the desired frequency with the first multi-phase bridge inverter, the efficiency of the charging process can be optimized. In this way, galvanic isolated charging with high efficiency is obtained, which allows for charging at up to half of the rated electrical machine power since the electric machine can be driven at the optimal speed and is not fixed to be driven at the line voltage frequency. The desired frequency can be created from any type of input, e.g. DC voltage, single-phase AC, multi-phase AC or from inputs having other types of voltage waveforms. In this way, it is also possible to use the same electronics to charge the battery with different line voltages. The line voltage can thus be both one phase and multi-phase and the line voltage may have different potentials and different frequencies. It is also possible to connect the electrical apparatus to a DC supply, which is common for charging electrical vehicles.
In the inventive method for charging a battery in an electrical apparatus, such as an electrical vehicle, the steps of applying a line voltage via a rectifying means creating a first DC link voltage from which the multiphase bridge inverter, that is connected from said DC link voltage to the first multi-phase stator winding, drives controlled currents through said winding such that the rotor is forced to rotate at the desired speed, such that the controlled current of the second multi-phase winding, that is connected to the second multi-phase inverter which has the battery to be charged in its DC link, generates the desired power flow to the battery, thus creating a charge current to the battery are comprised.
In this method, galvanic isolated charging with high efficiency is obtained by using two separate multi-phase bridge inverters connected to two separate stator windings in the electrical machine. Battery charging at half of the rated electrical machine power can thus be achieved.